Model of dMTMOTP and dPTMOTP pre-insertion complexes (A and C, respectively) and post-insertion complexes (B and D, respectively). DNA backbone is shown as blue ribbon, oxygen, sulfur, and phosphorous (of the triphosphates in pre-insertion complexes) are colored red, orange, and yellow, respectively. In the pre-insertion complexes, only the UBP is shown, and in the post-insertion complex the UBP and the templating nucleotide are shown.